1. Field of the Invention
The present invention relates to a casing used for industrial kilns, and more particularly to an improved casing for tunnel-shaped kilns.
2. Prior Art
A conventional roller hearth kiln known as an industrial kiln has a kiln made of refractory bricks in the form of a tunnel, which is housed or stored in a casing made of heat resisting steel. The casings are connected in the longitudinal direction of the kiln to form a tunnel-shaped kiln in order of several tens meters in total length usually. FIGS. 9(a), 9(b) and 9(c) in the accompanying drawings show a conventional casing unit. A casing unit 1 comprises horizontal frame members 2 provided in a longitudinal direction of the kiln, horizontal frame members 3 provided on the bottom of the kiln in an axial direction of rollers (not shown) and vertical frame members 4 vertically erecting at both ends of the horizontal frame members 3. Bottom plates 7 and side plates 5 are fixed to the inner side frame members 2, 3 and 4 along the outer periphery thereof by spot welding. Supporting fittings 6 are provided between the upper vertical frame members 4 and the lower vertical frame members 4. Rollers indicated by dotted line 8 in FIG. 9(c), are inserted in a width direction of the kiln. A plurality of rollers 8 are lined in a parallel relationship with each other in a longitudinal direction of the kiln.
However, a problem with such a conventional casing for kilns is that the total length of the kiln increases during firing operations. The side plates 5 and the bottom plates 7 adjacent in a longitudinal direction of the kiln and welded to each other expand themselves according to the high temperature, which causes accumulation in total length. As a result, the total length of the casing increases so that the distance between the kiln and the peripheral devices provided in front of and in the rear of the kiln is changed to cause troubles.
In addition, when the side plates 5 and the bottom plates 7 do not perform thermal expansion smoothly, the adjacent side plates 5 and the bottom plates 7 themselves compress and interfere with each other, resulting in deformation of the casing and dislocation of the kiln center, which often leads to misalignment of the kiln itself particularly when the temperature of the kiln is elevated or lowered. Distortion also occurs in the pipings or ducts around the kiln.